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John Phillips poses for a photograph December 1, 1980 with his daughter MacKenzie while in drug rehab at Fair Oaks Hospital in Summit, New Jersey.

When Mackenzie Phillips alleged in her book, "High On Arrival," that she and her father, John Phillips, had a 10-year sexual relationship, her former stepmother, Michelle Phillips, and half-sister, Bijou Phillips, quickly spoke out, expressing their doubts.

But now, in a new interview with Access Hollywood, Mackenzie's other half-sister, Chynna Phillips, is coming to her defense.

"I do believe her," Chynna told Access. "The details surrounding how it all went down, I can't validate because I was not there – and there were a lot of drugs."

The only sibling to stand by Mackenzie's claim of a decade-long incestuous relationship with her father John, the former Mamas & the Papas singer, Chynna told Access that while she knew of the relationship for years, the pain of it going public has been severe.

"It is a major revelation and it is something that I'm going to have to process in my own time and in my own way for many, many years," she said.

However, Chynna's mother and Mackenzie's former stepmom, Michelle, does not know whether to believe the affair, as she was quoted as saying, "Mackenzie is jealous of her siblings who have accomplished a lot and did not become drug addicts."

That is a claim which Mackenzie denied in an interview with Access' Nancy O'Dell in September.